The present invention relates to an image forming apparatus, such as a full-color copying machine or a color printer.
In the conventional art, a so-called 4-unit tandem full-color copying machine is known as an image forming apparatus capable of outputting a color image. In the full-color copying machine, four image forming units, which form toner images of yellow (Y), magenta (M), cyan (C) and black (B) on the basis of image signals obtained by color decomposition, are arranged along a conveyance belt.
The image forming unit for each color includes: a photosensitive drum which is in rolling contact with a conveyance belt; a charging device for charging the surface of the drum to have a predetermined potential level; an exposure device for forming an electrostatic latent image by exposing the surface of the drum to light; a developing device (a developing unit) for developing the electrostatic latent image formed on the surface of the drum by supplying toner thereto; and a transfer device for transferring the developed toner image to a recording sheet, which is carried in the adsorbed state on the conveyance belt. When the recording sheet adsorbed on the conveyance belt is made to pass by the four image forming units, toner images of the four colors are transferred onto the sheet in a superimposed manner. The recording sheet bearing the toner images is then supplied to a fixing unit, and this fixing unit fixes the toner images onto the recording sheet, thereby forming a color toner image.
The developing device of each image forming unit is provided with a toner sensor used for the sensing of density. A disposal toner hopper, which contains toner for toner replenishment, is attached to the developing device. The toner hopper has a toner replenishment roller, which is rotated when the developing device is replenished with toner.
Based on the density sensed by the toner sensor of each developing device, the developing device is replenished with toner supplied from the toner hopper, or a shortage of toner inside the toner hopper is determined.
Let us assume that in the four-unit tandem full-color copying machine described above, the toner density of the developing device of one of the four image forming units becomes low during a copying operation (more than A and less than C, A less than C). In this case, the developing device is replenished with toner from the toner hopper during the interval between two successive recording sheets, without the copying operation being brought to a halt. If the toner density becomes further lower, the toner replenishment performed in the interval between the two successive recording sheets may be insufficient. In this case, the developing device is replenished with toner from the toner hopper after the copying operation is brought to a halt. If the toner replenishment without the copying operation does not raise the toner density of the developing device to be more than a predetermined level (less than A), then the toner hopper is determined to contain no toner. In the manner described above, the toner-empty state is sensed without providing a remaining-toner-amount sensor inside the toner hopper.
According to the conventional art, when toner is supplied to a toner hopper of density of less than xe2x80x9cAxe2x80x9d without performing the copying operation, a developing device of different color is not replenished with toner. This being so, when the copying operation is resumed, it is likely that the toner density of another developing device becomes less than xe2x80x9cA.xe2x80x9d In this case, the copying operation has to be interrupted again, for toner supply. In short, the convention art entails the problem that the copying operation has to be sometimes interrupted so as to replenish a developing device with a toner hopper.
Accordingly, an object of the present invention is to prevent an image forming operation from being often interrupted for toner replenishment, by bringing the image forming operation to a halt and starting toner replenishment from the toner hopper in response to a low-density state of the developing device of one of a plurality of image forming units, and by simultaneously supplying toner to developing devices of other image forming units if those developing devices are in the low-density state as well.
To achieve this object, the present invention provides an image forming apparatus comprising: a plurality of developing means containing a developer inside and used for supplying the developer to an image bearer; a plurality of detectors for detecting densities of the developer in the plurality of developing means; replenishment means for replenishing the plurality of developing means with a developer based on detection results output from the plurality of detectors; first control means for executing a replenishment operation from the replenishment means to a developing means when detection by the plurality of detectors shows that the developer density is not more than a first density value in at least one of the developing means; second control means for executing a replenishment operation from the replenishment means to all the developing means whose developer densities are not more than the first density value until the developer densities exceed a predetermined density value, when detection by the plurality of detectors shows that the developer density is not more than a second density value, which is lower than the first density value in at least one of the developing means; and developer-empty state detecting means for detecting a developer-empty state of the replenishment means if the developer density of the developing means for which the replenishment operation is performed does not exceed the predetermined density value even after the replenishment operation by the second control means is executed for more than a predetermined period of time.
The present invention also provides an image forming apparatus comprising: a plurality of developing means containing a developer inside and used for supplying the developer to an image bearer; a plurality of detectors for detecting densities of the developer in the plurality of developing means; replenishment means for replenishing the plurality of developing means with a developer, based on detection results output from the plurality of detectors; first control means for executing a replenishment operation from the replenishment means to a developing means when detection by the plurality of detectors shows that the developer density is not more than a first density value in at least one of the developing means; second control means for executing a replenishment operation from the replenishment means to a developing means when detection by the plurality of detectors shows that the developer density is not more than a second density value, which is lower than the first density value in at least one of the developing means; third control means for executing a replenishment operation from the replenishment means to all the developing means whose developer densities are not more than the second density value until the developer densities exceed the first density value, when detection by the plurality of detectors shows that the developer density is not more than a third density value, which is lower than the second density value in at least one of the developing means; and developer-empty state detecting means for detecting a developer-empty state of the replenishment means if the developer density of the developing means for which the replenishment operation is performed does not exceed the first density value even after the replenishment operation by the third control means is executed for more than a predetermined period of time.
The present invention further provides an image forming method comprising: a detection step for detecting developing densities of a plurality of developing means each for supplying the developer to an image bearer; a first replenishment step for replenishing developing means from a developer replenishing means if the detection step shows that the developer density of at least one of the developing means is not more than a first density value; second replenishment step for executing a replenishment operation to all developing means whose developer densities are not more than the first density value, until the developer densities exceed a predetermined density value, when the detection step shows that the developer density is not more than a second density value which is lower than the first density value in at least one of the developing means; and a developer-empty state detecting step for detecting a developer-empty state of the replenishment means if the developer density of the developing means for which the replenishment operation is performed does not exceed the predetermined density value even after the replenishment operation is executed for more than a predetermined period of time.